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Compression Molding

What Is Compression Molding

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What is compression molding?

One of several molding forms; compression molding is the act of using compression (force) and heat to shape a raw material by means of a mold. In short, a raw material is heated until pliable, while the mold is closed for a certain time period. Upon removing the mold, the object may contain flash, excess product not conformed to the mold, which can be cut away.

Compression Molding Basics?

The following factors must be considered when using a compression molding method:

  • Material
  • Shape
  • Pressure
  • Temperature
  • Part thickness
  • Cycle Time

Plastics composed of both synthetic and natural materials are used in compression molding. Two types of raw plastics materials are most often used for compression molding:

  • Thermoset plastics
  • Thermoplastics

Thermoset plastics and thermoplastics are unique to the compression method of molding.  Thermoset plastics refer to pliable plastics that once heated and set to a shape may not be changed, while thermoplastics harden as a result of being heated to liquid state and then cooled.  Thermoplastics can be reheated and cooled as much as necessary.

The amount of heat required and necessary instruments to produce the desired product vary. Some plastics require temperature in excess of 700 degrees F, while others in the low 200 degree range.

Time is also a factor. Material type, pressure, and part thickness are all factors which will determine how much time the part will need to be in the mold. For thermoplastics, the part and the mold will need to be cooled to an extent, so that piece being manufactured is rigid.

The force with which the object is compressed will depend on what the object can withstand, particularly in its heated state. For fiber reinforced composite parts being compression molded, the higher the pressure (force), often the better the consolidation of the laminate, and ultimately the stronger the part.

The mold used depends on the material and other objects used in the mold. The three most common types of molds used in compression molding of pastics are:

  • Flash - requires accurate product inserted in the mold, removal of flash
  • Straight-does not require accurate product, removal of flash
  • Landed-requires accurate product, does not require removal of flash

It is important to ensure that no matter what material is used, the material covers all areas and crevices in the mold to ensure the most even distribution.

The process of compression molding begins with the material being placed into the mold. The product is heated until somewhat soft and pliable. A hydraulic tool presses the material against the mold. Once the material is set-hardened and has taken shape of the mold, an “ejector” releases the new shape. While some final products will require additional work, such as cutting away the flash, others will be ready immediately upon leaving the mold.

What is commonly made with compression molds?

Car parts and household appliances as well as clothing fasteners such as buckles and buttons are created with the help of compression molds. In FRP composites, body and vehicle armor is manufactured by means of compression molding.

Advantages of Compression Molding

Though objects can be made in a variety of ways, many manufacturers choose compression molding due to its cost-effectiveness and efficiency. Compression molding is one of the least expensive ways to mass-produce products. Furthermore, the method is highly efficient, leaving little material or energy to waste.

Future of compression molding

As many products are still made using raw materials, compression molding is likely to remain in widespread use among those seeking to make products. In the future it is highly likely that compression molds will use the landed model in which no flash is left when creating the product.

With the advancement of computers and technology, it is likely that less manual labor will be required to process the mold. Processes such as adjusting heat and time may be monitored and adjusted by the molding unit directly without human interference. It would not be far-fetched to say that in the future an assembly line may handle all aspects of the compression molding process from measuring and filling the model to removing the product and flash (if necessary).

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